The present invention relates to an ultrasonic diagnostic apparatus and, more particularly, to an ultrasonic diagnostic apparatus comprising a plurality of ultrasonic transducers having different ultrasonic characteristics. The ultrasonic characteristics include the resonance frequency of the ultrasonic transducer and the focal length of an ultrasonic lens mounted on the ultrasonic transducer.
When a typical ultrasonic diagnostic apparatus is being used, an ultrasonic transducer such as a piezoelectric element is generally positioned close to an object to be examined, a high-frequency AC voltage of a MHz-band equal to a resonance frequency of the transducer is applied to the transducer for an extremely short time period, so that the transducer is resonated, thereby causing it to emit an ultrasonic pulse. In this case, if the object to be examined is a uniform medium, the ultrasonic pulse propagates linearly therethrough. However, if there is a boundary is between tissues having different acoustic impedances, some pulses are reflected thereat, and some are transmitted therethrough. This reflected echo is received by the transducer, and the distance between the transducer and the boundary is measured in accordance with the speed of the ultrasonic wave and the time required for the ultrasonic pulse to reciprocate. When echo signals obtained from one transmitted pulse are aligned, an image signal representing slice information of the tissue along one direction is obtained. In a radial scanning type diagnostic apparatus, the transducer is rotated in a slice of the object to be examined. Therefore, by transmitting the ultrasonic pulses by the transducer n times when the transducer rotates once, an image signal in a scanning line is obtained by dividing a circle at n equal angular intervals, thereby obtaining a tomographic image of the object to be examined.
In this case, the amount of attenuation of the ultrasonic wave varies depending on resonance frequencies, and the distance range in which a clear image is obtained is predetermined also, depending on resonance frequencies. In general, the lower the resonance frequency, the farther the ultrasonic wave propagates. For this reason, in order to obtain a clear image in a wide range, from short to long distances, the total distance is divided into a plurality of short distances, and a transducer having an optimal resonance frequency is assigned to each short distance; i.e., a plurality of transducers must be provided.
Such a conventional ultrasonic diagnostic apparatus is disclosed in Japanese Patent Disclosure (Kokai) No. 61-11026. This apparatus comprises a probe having at its distal end a flexible tube to be freely inserted/extracted with respect to a body cavity, with two ultrasonic transducers having different resonance frequencies being incorporated in the distal end of the tube.
In this apparatus, in order to display an image in real time, by synthesizing close and remote images respectively obtained from the two transducers which are simultaneously driven, the frame rate must be decreased to 1/2. For this reason, this apparatus is not suitable for use in the diagnosis of a rapidly moving portion, e.g., the heart.
In addition, since a special synthesizing circuit is required to synthesize two images, the circuit arrangement is therefore complicated.
Furthermore, an ultrasonic echo signal from one of the transducers is undesirably superposed as noise on that from the other transducer. Additionally, when the resonance frequencies of the two transducers are different, if a frequency of a transmitted signal is matched with one of the transducers for the sake of simplicity of the device, the sensitivity is degraded because the frequency differs from the resonance frequency of the other transducer.
An external ultrasonic diagnostic apparatus has drawbacks similar to those of an internal ultrasonic diagnostic apparatus.
In addition, the probe comprising the ultrasonic transducers is generally independent from an electrical circuit portion for applying a drive signal (high-frequency AC voltage) to the ultrasonic transducers and for forming an image signal from an echo signal reflected by the transducers. The time required for applying the high-frequency AC voltage to the transducers, so as to transmit the ultrasonic pulse, varies in accordance with the respective resonance frequencies. However, since the electrical circuit portion is expensive, a single electrical circuit portion is used for the two transducers in the apparatus disclosed in the above reference. For this reason, in accordance with the type of transducer connected thereto, the high-frequency AC voltage cannot be applied to the ultrasonic transducers within an optimal time interval. If the ultrasonic transducer is not driven for a time according to the resonance frequency, vibration of a mode other than the mode of vibration in the direction of thickness, e.g., the mode of vibration in the lateral direction or longitudinal direction or distortion vibration will be generated.
A conventional example for solving the above drawbacks is disclosed in Japanese Patent Disclosure (Kokai) No. 61-37145. In this apparatus, the frequency of a transmitted pulse can be selected from among a plurality of frequencies. However, in this conventional example, since a plurality of types of transmitted pulses having different frequencies are generated and one of them is selected in accordance with the type of transducer, useless transmitted pulses are generated, and the circuit cannot be made compact.